1. Field of the Invention
The present invention relates generally to a data transmission system in a wireless communication system and a method thereof, and more particularly to a system for transmitting data by applying a band AMC (Adaptive Modulation and Coding) in an OFDMA (Orthogonal Frequency Division Multiplexing Access) system and a method thereof.
2. Description of the Related Art
Typically, a wireless communication system is a system developed to be used in cases in which a fixed wire network cannot be connected to a terminal. The wireless communication system has been developed into a mobile communication system such as a cellular system. The cellular system connects a base station, which communicates with a terminal through a wireless channel, to a wire network. A representative cellular system is a cellular mobile communication system that uses a CDMA (Code Division Multiple Access) system.
Although the cellular system was developed to provide voice communications, systems that can provide diverse data services have now been proposed. Additionally, since the amount of data required by respective users is increasing, users now desire to transmit a larger amount of data at a higher speed. Accordingly, research has been made to support high speed data transfer in the cellular system that uses the CDMA system.
Meanwhile, in order to provide a large amount of data to users at a high speed, research into an OFDM (Orthogonal Frequency Division Multiplexing) system that is different from the CDMA system has actively been in progress, in order to commercialize a system that uses the OFDM system. Accordingly, the IEEE 802.16 standardization group of IEEE (Institute of Electrical and Electronics Engineers) is carrying out the establishment of the IEEE 802.16d standard for proving a broadband wireless Internet service with respect to a fixed terminal.
The OFDM system may be defined as a second-dimensional access method that combines the techniques of TDA (Time Division Access) and FDA (Frequency Division Access). In IEEE P802.16d/D3-2004, an OFDMA (Orthogonal Frequency Division Multiple Access) method is used in which the entire band is divided into subcarriers, some of these subcarriers are grouped into subchannels, and the respective subchannels are allocated to all users. Accordingly, all user terminals in the system use the subchannels composed of subcarriers broadly spread over the entire band, and in the system that uses the OFDMA system (hereinafter referred to as the ‘OFDMA system’), each of user data is transmitted to corresponding user through a plurality d of subcarriers that constitute a specified subchannel.
Meanwhile, in the broadband OFDMA system, all users connected to a base station share and use common channels, and sections used by the respective users are allocated for each frame by the base station. Accordingly, the base station divides access information into uplink access information and downlink access information, and allocates the uplink access information and the downlink information to the front part of each frame to be broadcast to all users.
In the broadband OFDMA system, the access information that is transmitted for each frame includes information such as a modulation method, a coding rate, etc. A general frame structure will now be explained with reference to the accompanying drawings.
FIG. 1 is a view illustrating a frame structure including uplinks and downlinks in a general broadband OFDMA system. In FIG. 1, the vertical axis represents the numbers (S, S+1, S+2, . . . , S+L) 147 of various subchannels, and the horizontal axis, i.e., the time axis (t), represents the numbers 145 of OFDMA symbols.
Referring to FIG. 1, a frame is composed of a downlink (DL) 149 and an uplink (UL) 153 divided in time, and an OFDMA frame is composed of a plurality (for example, L) subchannels. This OFDMA system is a system having an object of obtaining a frequency diversity gain by dispersing the whole subcarriers used in the system and especially data subcarriers over the entire frequency band.
The downlink 149 includes a preamble 111, located in front, for synchronizing a transmitter-side terminal with a receiver-side base station, broadcast data information such as FCH (Frame Control Header) 113, DL_MAP 115 and UL_MAP 117, and downlink bursts 121, 123, 125, 127 and 129.
The uplink 153 includes preambles 131, 133 and 135, located in front of uplink bursts 137, 139 and 141, for synchronizing the transmitter-side terminal and the receiver-side base station, and a ranging subchannel 143 for adjusting a receiving power of the base station. The base station informs the terminal of information about positions and allocations of the uplink bursts 137, 139 and 141 and the downlink bursts 121, 123, 125, 127 and 129 through DL_MAP 115 and UL_MAP 117. The terminal is variably allocated with the subchannels in which the frequency and the symbol are combined for each frame through this information, and communicates with the base station. That is, the terminal receives and uses different subchannels, not fixed subchannels, for each frame.
Additionally, a conversion process from the downlink to the uplink is performed for a first TTG (Transmit/receive Transition Gap) 151, and a conversion process from the uplink to the downlink is performed for a second RTG (Receive/transmit Transition Gap) 155. After these conversions are processed, a preamble region is placed so that the terminal can obtain the system sync.
As described above, the frame is coded, modulated and then transmitted in accordance with the most accurate coding rate and transmission rate before it is transmitted through a wireless channel. For example, the channel state of the terminal is changed according to the cell position of the terminal. A terminal located in the center of the cell has a good channel state, and a high-efficiency modulation and a low coding rate are used, while a terminal located on the cell boundary has a poor channel state due to interference with adjacent base stations, and a low-efficiency modulation and a high coding rate are used, so that the reliability of data can be heightened.
However, since the conventional OFDMA system uses subchannels composed of subcarriers broadly spread over the whole band, it is impossible for a specified terminal to use a frequency band determined to have a good quality according to its channel state or to change the coding or modulation method according to the channel quality.
Additionally, the frame structure as described above makes it difficult to use a multiplexed coding method by which the coding and modulation being used is changed by frequency bands. This is because if the multiplexed coding method by frequency bands is used in the conventional system, which groups the subcarriers and uses the subcarrier groups as the subchannels, subfrequency bands obtained by dividing a frequency band are used as the subchannels.
Accordingly, the subchannels of the multiplexed coding method by frequency bands cannot be expressed by the above-described general frame structure and the MAP information elements, which define burst allocation elements, included therein, for informing the terminal of information about the coding and modulation.